In coal-fired boilers of the type typically used to generate electricity, coal is pulverized in mills and then transported pneumatically via heated primary air through burner feed pipes to burners within the boiler. A typical large coal-fired boiler will operate with about seven pulverizer mills and 40 burners. The mills pulverize the feed coal to a particle size of about 75% through a 75 .mu.m screen, degrading to about 40% through a 75 .mu.m screen when the mill becomes worn. The coal-air mixture is blown out of the mill into about six burner feed pipes of diameter 300 to 600 mm at velocities of about 25 m/sec. The coal/air mass ratio in these feed pipes is about 0.8.
A significant problem for large coal-fired boiler operation arises from non-uniform splitting of the pulverized coal between the burner feed pipes. This can lead to localized areas of incomplete combustion, slagging and fouling, increased NO.sub.x emissions and a reduction in boiler efficiency.
In present practice, the coal flow rate into the pulverizer mills can be monitored with gravimetric feeders. The primary air flow rates from each mill are balanced before coal is added using standard pitot tube flow meters. However this method does not measure coal loading and continuous readings during boiler operation are not feasible.
There is a clear need for a reliable and accurate on-line instrument to measure to within about 5% the relative mass flow rates of pulverized coal in feed pipes into large coal-fired boilers. The measurement technique should preferably be non-invasive, accurate and relatively inexpensive.
A Patent Co-operation Treaty application published under no. WO 87/05696 and U.S. Pat. No. 4,882,934 describe meters in which a narrow ultrasonic beam (angular spread 4.degree.) of frequency 460kHz is transmitted at 90.degree. to the flow direction and flow velocity is determined from the downstream drift of the beam which is measured by physically moving the ultrasonic receivers. Solids loading is measured from the mean attenuation of the transmitted ultrasonic beam. The main disadvantage of this technique is the need to constantly move the ultrasonic receivers to determine the peak signal amplitude of the highly fluctuating partly attenuated signal. The signal is highly fluctuating partly because of the narrow ultrasonic beam which must be used in this system. Typically about 50 pulse amplitude measurements are required at each of 30 receiver locations, making a total of 1500 measurements for each determination of coal mass flow. The technique is therefore relatively complex and it has difficulty in measuring rapid changes in velocity and mass loading. As well, the downstream drift distance used to measure velocity is affected by changes in gas temperature and a correction is required.
The British Coal Utilisation Research Association (BCURA) has developed and field tested a pulverized coal mass flow meter comprising an ultrasonic gas velocity meter combined with a beta-particle absorption meter for coal density. The ultrasonic meter comprises two pairs of ultrasonic transducers with 40kHz continuous ultrasonic waves transmitted upstream and downstream at about 45.degree. to the flow direction. The gas velocity was derived from the phase difference between the two received signals. The BCURA pulverized fuel mass flow meter has been evaluated and, although it has some problems, it has been shown to be capable of measuring the mass flow to individual burners to within about .+-.10%. The BCURA mass flow meters have also been field tested at a British power station. However there was sufficient erosion and build-up on the sensor faces in these field tests that the mass flow meter was not recommended for industrial use. In practice, the major disadvantage of the BCURA meter is the hazard of using radioactive beta-ray sources separated from an abrasive and hostile atmosphere by only thin windows.